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4.9L rough idle

  • vaccum leak?

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  • Spark?

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  • fuel?

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Discussion Starter #1
New: Spk plugs, wires, Dist. cap, rotor, coil,TPS, Vaccum lines/connectors, filters, Mobile 1, Only Chevron Premium.
Performed Top Engine Cleaner as directed.
Still it "misses", or rough idles. It Is perfectly smooth at startup for about a minute. Also, it is better when in gear and A/C is on.
My guess is some hidden vacc. leak (idles @ 40-41)
or plug wires arking.
 

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'05 Expedition
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Don't know this for sure, but it seems like whenever something like this comes up, it turns out to be some 02 sensor failing somewhere.

The cold start info you gave is what points me in that direction.
 

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2003 Cadillac DeVille
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397 Posts
Next things you might wanna check are the ignition timing, and the distributor itself.

O2 sensor would not cause a miss and would set a code. Try running diagnostics and check for codes.
 

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01 frontier , 89 Shelby CSX vnt
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anyone see that he replaced his TPS ? wonder if hes done idle relearn procedure?

i also wonder about his EGR tubes under the throttle plate being clogged or worse his egr valve not working

i also wonder about his fuel pressure at idle , wonder what the spec is there

looks like me and 2dfx have another sibling here
 

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Discussion Starter #5 (Edited)
2Clown2 said:
New: Spk plugs, wires, Dist. cap, rotor, coil,TPS, Vaccum lines/connectors, filters, Mobile 1, Only Chevron Premium.
Performed Top Engine Cleaner as directed.
Still it "misses", or rough idles. It Is perfectly smooth at startup for about a minute. Also, it is better when in gear and A/C is on.
My guess is some hidden vacc. leak (idles @ 40-41)
or plug wires arking.

Fixed rough idle. Capped ALL of the vaccum outlets except the Vacc. Sensor, and put a new rotor cap on. Now it's "Pinging" sometimes.
Gas is Chevron premiun and a bottle of O'Riely octaine booster. Can anybody tell me why it's pinging ?
No codes. One O2 sensor cycles a lot faster. Idle re-learn proceedure done.
 

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01 frontier , 89 Shelby CSX vnt
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probably the egr system , i see alot of 4.9 pining issues resoved with the cleaning fo the egr tubes under the throttle plates and if it still pings, replecment of the egr valve , there is a cadillac "how to" message board but i dont have the link on me , the caddy 4.9 doesnt have a knock sensor so the ecu doesnt know if its pingin or not , instead it comes up with a crude idea on pinging based on readings from sevreal sensors , it uses the egr system to keep the engine from pinging
 

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2Clown2 said:
New: Spk plugs, wires, Dist. cap, rotor, coil,TPS, Vaccum lines/connectors, filters, Mobile 1, Only Chevron Premium.
Performed Top Engine Cleaner as directed.
Still it "misses", or rough idles. It Is perfectly smooth at startup for about a minute. Also, it is better when in gear and A/C is on.
My guess is some hidden vacc. leak (idles @ 40-41)
or plug wires arking.

I recently had a COMPLETE tune up. ran well for a few days and then started stalling and bucking.

Popped the hood and looked around. Noticed the garage hadn't hooked up a vac line to the cannister.

Simple fix for me. Hopefully for you too.
 

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Mine is doing the same thing. I guess I need to get in there and look at things. I honestly don't remember it acting that way until I cleaned the TB and did the idle relearn.

Maybe I need to blank the computer memory overnight and do the relearn?
 

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Discussion Starter #9
ga_caddy said:
Mine is doing the same thing. I guess I need to get in there and look at things. I honestly don't remember it acting that way until I cleaned the TB and did the idle relearn.

Maybe I need to blank the computer memory overnight and do the relearn?

Found it ! Remove the vacc line to the cruise, evap solenoid, vaccum reservoir, and cap the 1/4' vaccum pipe to the Throttle Body. It's the one on the psngr side to the upper rear. It angles up slightly. (See Attachment) If this smoothes out the idle then thats where the problem lies. I found several rubber hoses that were carbonized! Likely the evap hose !
 

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Discussion Starter #10
2Clown2 said:
Fixed rough idle. Capped ALL of the vaccum outlets except the Vacc. Sensor, and put a new rotor cap on. Now it's "Pinging" sometimes.
Gas is Chevron premiun and a bottle of O'Riely octaine booster. Can anybody tell me why it's pinging ?
No codes. One O2 sensor cycles a lot faster. Idle re-learn proceedure done.
Found it ! Removed the vacc line to the cruise, evap solenoid, vaccum reservoir, and capped the 1/4' vaccum pipe to the Throttle Body. It's the one on the psngr side to the upper rear. It angles up slightly. (See Attachment) This smoothed out the idle, so thats where the problem lies. I found several rubber hoses that were carbonized! Likely the evap hose was the one!
 

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2Clown2 said:
Found it ! Removed the vacc line to the cruise, evap solenoid, vaccum reservoir, and capped the 1/4' vaccum pipe to the Throttle Body. It's the one on the psngr side to the upper rear. It angles up slightly. (See Attachment) This smoothed out the idle, so thats where the problem lies. I found several rubber hoses that were carbonized! Likely the evap hose was the one!

I figured that might be it. I checked all the vac lines today and the one to the cruise/ac/evap is just about gone. I bought some new vac line and an EGR Valve gasket this evening. I plant to remove the EGR and clean everything there then replace the vac lines.

Thanks for the help!!

Now I just have to figure out what to do with the heater core/evap box that is falling apart.
 

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Discussion Starter #12
Rough Idle/Vaccum leak/Evap line to gas tank

Traced the vaccum leak to the line from the gas tank to the Evap canister. Looks good up arround the cannister so it must be either under the car or where it goes into the tank. Do I need to drop the tank? Anyone had this? My Alldata diagrams aren't satisfying. (See Attach.)
 

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Discussion Starter #13
wildoaf said:
I recently had a COMPLETE tune up. ran well for a few days and then started stalling and bucking.

Popped the hood and looked around. Noticed the garage hadn't hooked up a vac line to the cannister.

Simple fix for me. Hopefully for you too.

That was it! Been looking for this leak for too long! Mine must be either under the car or at the gas tank. What finally proved it for me was that I've never heard vaccum release when opening the gas cap.
Thanks.
 

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2Clown2 said:
That was it! Been looking for this leak for too long! Mine must be either under the car or at the gas tank. What finally proved it for me was that I've never heard vaccum release when opening the gas cap.
Thanks.
The Vapor line from the tank to the canister should not have vacuum. It simply goes into the Charc Canister where the vapors and any condensed fuel are stored until the canister purge Sol opens to allow vacuum to purge the vapors. If you are getting vacuum in your tank you need to clean the tank vent valve. This is a hose that is attached to the Canister and then is clamped to the cooling fan support. You take the cap off the hose, blow thru the cap to make sure it's clean and put it back.

You should also test the Charc Canister and Purge Sol for proper operation.
 

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my 4.9 can be rough at an idel.......but as soon as the rpm comes it, she smooths out and purrs really nice......it's just one of those 4.9 things ya know? since there isn't much that goes wrong with these enignes, we gotta have something.....it gives the car that nice classic muscle car rumble at idel attitude :D
 

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Night Wolf said:
my 4.9 can be rough at an idel.......but as soon as the rpm comes it, she smooths out and purrs really nice......it's just one of those 4.9 things ya know? since there isn't much that goes wrong with these enignes, we gotta have something.....it gives the car that nice classic muscle car rumble at idel attitude :D
Well that is when mine is doing. The EGR Valve was broken and the passages were blocked but that is all fixed. All the vacuum lines were replaced. The TPS got replaced because I broke it. There was a vacuum leak at the rubber hoses on either end of the PCV line.

The idle is a lot smoother now but I am still not happy. I'm going to take it to a friends shop sometime soon to put it on the scope. Hopefully when that is done we'll be able to pinpoint the cause. I have a funny feeling it's fuel related.........
 

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Discussion Starter #17
Took the Vapor line off at the Canister and applied vacuum to the tank. It will not hold any. How does the gas tank vent, if this Vapor line is the only line to it?
I will test the Canister and Purge Sol first thing.
 

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2Clown2 said:
Took the Vapor line off at the Canister and applied vacuum to the tank. It will not hold any. How does the gas tank vent, if this Vapor line is the only line to it?
I will test the Canister and Purge Sol first thing.
OPERATION
The Evaporative Emissions (EVAP) solenoid receives 12 volts from the fuse panel. The Powertrain Control Module (PCM) energizes the EVAP solenoid by grounding CKT 428. When the solenoid is energized, it allows the canister to purge. When a purge mode is commanded, stored fuel vapor is drawn into the inner dip tube through a calibrated orifice at the base of the tube. Any liquid trapped in the liquid well is also drawn into the inner dip tube through a smaller calibrated orifice. Fresh air is drawn into the canister at the top mounted vent.

Under cold engine, "OPEN LOOP," or idle conditions, the solenoid is de-energized by the PCM, which closes the purge solenoid and blocks vacuum to the canister.

The canister will be enabled (PCM energizes the solenoid) when:


Coolant is above 80°C (176°F).
Closed loop has been achieved for at least 30 seconds.
The throttle switch is open.
Vehicle speed greater than 10 mph.
Engine speed above a threshold. OR
Code P013, P017, P042, P043, P044, or P045 is present.
If the solenoid is closed, (not receiving power or has a stuck plunger), the canister will not purge to the intake manifold. This will prevent the canister from purging fuel vapors and could result in a complaint of fuel odors coming from the engine compartment.

FUNCTIONAL TEST



Attach a hose to the fuel vapor port (from fuel tank) of the canister and attempt to blow through it. Air should pass into canister, If not, replace the canister.
Disconnect the EVAP solenoid electrical connector, attach a hose to the upper port of the canister, and attempt to blow through it. Air should not pass into the canister. If air passes, replace the canister.
Reconnect all purge hoses and the solenoid electrical connector.
 

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Discussion Starter #19
If I listen real carefully and watch the RPM's (PO11),trying to hold a steady 1500rpm, I can tell that it is doing the idle thing through-out the range. This cannot be good for the engine. The EVAP cannister checks good, so I too am ready to accept this rascall as "character" and live with it!
 
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